Combined oil cracking and viscosity breaking process



H. V. SMITH mmw, 5

Filed Sept. 11, 1936 May 3, 1938.

COMBINED OIL CRACKING AND VISCOSITY BREAKING PROCESS Patented May 3, 1938 PATENT: oFrIcE CUMBINED OIL CRACKING AND VISCOSITY BREAKING- PROCESS Howard V. Smith, Eldorado, Kans., assignor to Skelly Oil Company, Tulsa, Okla.

Application September 11, 1936, Serial No. 100,366

9 Claims.

My invention consists in new and useful improvements in a combined oil cracking and viscosity breaking process in the nature of the socalled stage or step cracking operations.

For some years now, one of the trends in the oil cracking industry has been generally along the lines of this stage or step cracking, which means that the lighter elements of the charging stock which do not so readily form carbon, are subjected to drastic cracking conditions to convert them into gasoline, while the heavier elements of the charging stock which more readily form carbon are subjected to a mild cracking action which is instrumental in converting a large part of them into lighter cracking stock. This lighter cracking stock may then be subjected to the drastic cracking conditions without danger of carbon formation.

The mild cracking of a heavy stock is known in the industry as a viscosity breaking operation as it was originally employed to reduce the viscosity of heavy asphaltic crude oil residues so as to convert them into marketable fuel oil. However, the purpose of the present day viscosity breaking is to convert as much as possible of the high boiling material into charging stock with a lower gas oil boiling range in order to produce cracking stock which may be subjected to a gasoline producing operation, or high temperature conditions, without excessive coke formation.

A large amount of heat is required to bring the charging stock for a viscosity breaker up to the viscosity breaking temperature, and this heat must be acquired at a high temperature level; On the other hand, the hot cracked products from a high temperature cracking coil contain a large amount of waste heat at a high temperature level, which heat is difficult to recover by indirect heat exchange.

It is therefore an object of my inventionto mixthe viscosity breaker charging stockwith the hot stream from the high temperature cracking coil to accomplish an economical heat exchange and more efficient utilization of waste heat. This mixture may then be readily supplied with the small amount of additional heat required and, given the necessary time factor, will accomplish the desired amount of viscosity breaking.

It is a further object of my invention, after mixing the viscosity breaker charge with the hot stream from the cracking coil, to conduct the combined stream to a vapor separating device operated under high pressure, where residual vapors are separated from the charging stock for the viscosity breaking coil, thus relieving the viscosity breaking coil of an unnecessary burdenand increasing its capacity,

Furthermore, by thus operating the vapor separating device under a pressure equal to or greater than the pressure in the viscosity break ing coil, I eliminate the necessity of employing a pump to force the mixture through said coil.

Finally, it is the object of my invention to provide a process of this character which will operate to produce greater yields of gasoline from a given quantity of charging stock, with an increased capacity onthe cracking still, and one which is effective for obtaining a gasoline of high octane number.

With the above and other objects in view which will appear as the description proceeds, my invention resides in the novel features herein set forth, illustrated in the accompanying drawing and more particularly pointed out in the appended claims.

Referring to the drawing,

Fig. 1 shows diagrammatically an operation carried out under the teaching of my invention.

Fig. 2 is an enlarged sectional view of the high pressure vapor separator, and

Fig. 3 is a top plan view of the same.

In the drawing, I represents a charging pump in feed line 2, the latter leading to a preheating coil 3 located in a conventional bubble tower 4. The discharge end of the preheating coil 3 leads into line 5 which terminates in communication with a vapor separator 6 provided with a series of suitable bafiies l and fractionating trays 8 in its upper portion. A false bottom 9 is preferably provided in the lower portion of the vapor separator 6 dividing the same into two separate zones in and IL. The lower extremity of the upper zone It] is in communication with a high pressure vapor separator l2 through line l3, having interposed therein a hot oil pump M, the upper portion of the high pressure separator l2 discharging back into zone ID of separator 6 through line Hi. This line if: is provided with a control valve IB which is actuated by a liquid level controller ll operatively attached to the separator l2.

The details of structure of the high pressure separator [2 will best be seen from Figs. 2 and 3 where l8 represents a cylindrical vessel which may be approximately from two to four feet in diameter, with relatively thick Walls and having a tangential inlet l9 near its upper extremity. The lower portion of the vessel l8 tapers into an outlet 20 at the bottom, and the top of the vessel is closed by a head 2| suitably secured to radial flanges 22. 23 represents a centrally disposed vapor pipe open at its lower extremity, which is disposed midway of the vessel I8, the upper end of said vapor pipe being in communication with a vapor discharge pipe 24 extending through the head 2|. Suitable flanges are provided on the tangential inlet I9, the liquid outlet 20 and vapor outlet 24 to facilitate the connection of these points into the system.

Returning now to Fig. 1, a line 25 leads from the outlet 26 of high pressure vapor separator I2 to the inlet of a viscosity breaking coil 26 located in furnace 27, the discharge end of said coil being connected by line 28 to the lower portion of zone I I of the first mentioned vapor separator 6, a throttle valve 29 being located in line 28. A line 30 leads from the bottom of zone II to a small vessel 3|, the latter discharging through line 32 and cooling coil 33 to a suitable storage point not shown. The upper portion of zone I I is in communication with the lower portion of zone II] by means of line 34. I

A line 35 leads from the top of vapor separator 6 through a heat exchanger 36 and into the lower portion of the bubble tower 4, the top of which discharges through line 31 and condenser 38 into a gas separator 39 having a gas discharge line 40 at its upper end, and a liquid discharge line M at its lower end. The lower portion of the gas separator 39 is connected by line 42 back into the upper portion of bubble tower 4, a suitable pump 43 being provided in the line 42. A line 44 and cooling coil 45 may lead from an intermediate point in the bubble tower.

The bottom of the bubble tower 4 is provided with a discharge line 46 which branches off into line 41 provided with pump 48, said line 41 passing through the heat exchanger 36 to the inlet end of high temperature cracking coils 48 in the furnace 21. Through line 49, the discharge end of cracking coils 48 by-passes the viscosity breaker 26 and communicates with the inlet end of a top bank of coils 50, the latter being in communication with a series of roof tubes 5| and floor tubes 52. From the floor tubes 52 a line 53 leads to a final bank of coils 54 interposed between the viscosity breaking coils 26 and the top bank of coils 50, said coils 54 discharging through line 55 into line I3 leading to the high pressure vapor separator I2.

Another branch line 56 leads from line 46 from the bubble tower 4 to line 28 entering the lower zone II of vapor separator 6, a suitable pump 51 being provided therein.

The lower portion of the bubble tower 4 may be in communication with a vessel 58 by means of line 59, said vessel discharging at its bottom through line 60 and pump 6I into the upper portion of the vapor separator 6, suitable jets 62 being provided in the top of said vapor separator. A line 63 leads from a low point in the vapor side of heat exchanger 36 to the vessel 58.

Having thus described an example of a set-up by means of which my improved process may be carried out, its operation is as follows:

The fresh charge, which may be gas oil, topped crude or any desired type of cracking stock, is pumped into the system by pump I through line 2, entering the preheater or heat exchange coil 3 in the bubble tower 4. In this preheater, the charge is raised to a temperature of approximately between 500' F and 550 F. and it then passes through line 5 into the upper zone ID of vapor separator 6 where it commingles with the hot vapors from the cracking still, the baffle plates I facilitating the commingling of these products. The liquid collected in zone I 0 above the false bottom 6 is discharged through line I3 and forced by the hot oil pump I 4 into the upper portion of the high pressure vapor separator I2.

Due to the tangential inlet I9 into which line I3 discharges, a centrifugal motion is set up in the separator I2 and the vapors separated therein pass out through line I5 and control valve I6 back into zone II] at a point above the discharge line I3. The control valve I6, as before stated, is actuated by a liquid level controller I! attached to the high pressure vapor separator I2, thereby automatically regulating the return of vapors from the high pressure separator I2 to the first separator 6. I have found that appropriate temperatures for the material in the high pressure vapor separator I2 are from 825 F. to 900 F., but even higher temperatures may be used on occasions. A suitable pressure range for this vapor separator is from 100 to 800 pounds.

From the bottom of the vapor separator I2, the separated liquid enters line 25 and passes through the viscosity breaking coil 26 in furnace 2'! where it is heated to temperatures of from 850 F. to 950 F. The oil passes from the viscosity breaking coil through line 28 and throttle valve 23 into the lower zone II of vapor separator 6 in which the fuel oil is separated from the vapors and passes out into the small vessel 3|, and from thence through line 32 and cooling coil 33 to storage. The liquid level in the vessel 3i is kept sufficiently low that no level of liquid accumulates in the zone II, and in this way the tar or fuel oil remains in the vapor separating zone for a very short time. If necessary, the temperature in the zone I I may be reduced by circulating some gas oil from the bottom of the bubble tower 4 through the line 46, the pump 51 and the line 56.

The vapors from this zone I I pass out through line 34 into the lower portion of zone I6 where they commingle with the vapors from the high pressure vapor separator I2 and the fresh charge oil.

The commingled vapors in the zone If! of the vapor separator 6 pass upwardly, contacting the baflles I, through fractionating trays B which are cooled by a stream of reflux sprayed in through nozzles 62, and the fractionated vapors pass out through line 35 and heat exchanger 36 into the lower portion of the bubble tower 4 where said vapors are fractionated in the conventional manner. By reason of the fractionation which takes place in the-zone II] of vapor separator 6, the

heavier constituents of the vapors are condensed and fall back to pass through the viscosity breaking system, and the gas oil condensate which collects in the bottom of the bubble tower 4 is a clean cracking stock which may be subjected to very high cracking temperatures without excessive carbon formation.

Condensate which forms in the heat exchanger 36 from the vapors, passes out through the line 63 into the vessel 58, where it mixes with gas oil from bubble tower 4 entering through line 53, and passes out through line 60 and pump SI through the sprays 62 into the top of the vapor separator 6.

Cracked gasoline and gas pass out from the top of the bubble tower 4 through line 31 to condenser 38, and from thence to gas separator There the uncondensable gases are discharged through line 40 and the gasoline through line 4! to suitable storage. A quantity of gasoline may be returned by line 42 and pump 43 to the top the bubble tower t through the line 44 and cooling coil 45, if desired.

The gas oil condensate collects in the bottom of the bubble tower 4 and is discharged through line 46 and forced by pump 48 through branch line H, through heat exchanger 36 into the lower bank of cracking coils 48 in furnace 27, and from thence through line 49 into the top bank of coils 59, through the roof tubes 5|, the floor tubes 52, and finally through the intermediate bank of cracking coils 54, from whence it is discharged into line 53 leading to the high pressure vapor separator. i2. During its passage through the high temperature cracking coils, the oil is prefera bly heated to a temperature of from 925 F. to over 1000 F.

It will be apparent that in the operation of separator I2, suiiicient oil should be maintained in the lower part of the separator to actuate the float which in turn controls the valve V3 for re leasing the vapors into the fractionating system,

and that in a small separator of this type, there is insufiicient space to afford the necessary time factor for accomplishing cracking of either vapor or liquid. Therefore, any further cracking of the mixture of hot cracked products and reflux condensate entering the high pressure separator 12 is minimized during the separation step.

It will thus be seen that with my improved combined cracking and viscosity breaking process, with the high pressure vapor separating step interposed between the point of mixing of the cracked stream with the heavy oil stream, and the viscosity breaker, a large part of the light products produced in the vapor phase operation are removed from the stream and do not pass through the viscosity breaking coil, which relieves a needless burden on the system and affords a far greater capacity for viscosity breaking. in a coil of a given size.

Furthermore, it will be apparent that the greater yields of gasoline produced by my process are obtained by the viscosity breaking operation which converts a maximum proportion of the charging stock into clean cracking stock, and a minimum proportion into tar. In this particular type of viscosity breaking operation, a maximum of viscosity breaking may be readily obtained because the high temperature at which the charge enters the viscosity breaking coil makes necessary the addition of only a small amount of heat through the walls of the tubes, a condition which minimizes any coking tendency in the tubes.

The increased capacity obtainable by my process is the result of the eflicient utilization of the heat in the cracking stream as hereinbefore de scribed, and the operation of the cracking coil on a clean gas oil stock which permits high rates of heat input into the tubes without danger of carbon formation.

As before stated, my process is effective for obtaining a gasoline of high octane number, and it is to be noted that this is the result primarily of high cracking temperatures to which the clean gas oil stock can be subjected. The viscosity breaking operation produces a small amount of gasoline but it has been found that when the viscosity breaking operation is conducted at temperatures near 900 F. or higher, this gasoline has a very satisfactory octane number.

From the foregoing it is believed that the advantages and operation of my process may be readily understood by those skilled in the art without further description, it being borne in mind that numerous changes may be made in the details disclosed without departing from the spirit of my invention as set out in the following claims.

What I claim and desire to secure by Letters Patent is:---

1. The method of treating hydrocarbon oil comprising cracking a clean oil distillate, discharging the cracked distillate into a high pressure separating zone admixed with reflux condensate from a refluxing zone maintained at a lower pressure than said separating zone, to rapidly separate vapors from liquids while minimizing further cracking, subjecting the separated hot vapors to reflux condensation with an incoming stream .of heavier charging oil in said refluxing zone, to form said reflux condensate, subjecting the liquid from said separating zone to a mild cracking, further separating the products of said mild cracking, removing tar from the system, fractionating the vapors from said refluxing zone and said further separation, and condensing and collecting the fractionated vapors.

2. The method of treating hydrocarbon oil comprising cracking a clean oil distillate, discharging the cracked distillate into a high pressure separating zone admixed with a stream of reflux condensate from a refluxing zone maintained at a lower pressure than said separating zone, to rapidly separate vapors from liquids without substantial further cracking, subjecting the separated hot vapors to reflux condensation with an incoming stream of heavier charging oil in said refluxing zone, to form said reflux'condensate, fractionating the vapors from said refluxing zone, subjecting the liquid from said separating zone to a mild cracking, further separat ing the products of said mild cracking, removing tar from the system, subjecting the vapors from said further separation to refluxing action in said refluxing zone and condensing and collecting the vapors from said fractionating zone as the desired product.

3. The method of treating hydrocarbon oil com.- prising cracking a clean oil distillate, discharging the cracked distillate into a high pressure separating zone admixed with reflux condensate from a refluxing zone maintained at a lower pressure than said separating zone, to rapidly separate vapors from liquids without substantial further cracking, subjecting the separated hot vapors to reflux condensation with an incoming stream of heavier charging oil in said refluxing zone, to form said reflux condensate, subjecting the liquid from said separating zone to a mild cracking, further separating the products of said mild cracking, removing tar from the system, jointly fractionating the vapors from said refluxing zone and said further separation to form said clean oil distillate, and condensing and collecting said fractionated vapors.

4. A process of treating heavy hydrocarbon oils to produce lighter hydrocarbons and gasoline, which includes preheating a stream of said heavy hydrocarbons, separating vapors from liquids in a primary separating zone, conducting the separated liquid to a second separating zone maintained at a pressure higher than said primary separating zone, admixed with a stream of hot cracked products from a severe cracking zone, rapidly separating vapors from liquids in said second separating zone without substantial further cracking, cracking the liquids from said second separating zone in a less severe cracking zone, separating the vapors and liquids resulting from said less severe cracking, removing tar from the system, and subjecting the vapors from all said separating zones to reflux condensation in a common refluxing zone.

5. A process of treating heavy hydrocarbon oils to produce lighter hydrocarbons and gasoline, which includes preheating a stream of said heavy hydrocarbons, separating vapors from liquids in a primary separating zone, conducting the separated liquid to a second separating zone maintained at a pressure higher than said primary separating zone, admixed with a stream of hot cracked products from a severe cracking zone, rapidly separating vapors from liquids in said second separating zone without substantial further cracking, cracking the liquid from said second separating zone in a less severe cracking zone, separating the vapors and liquids resulting from said less severe cracking, removing tar from the system, subjecting the vapors from all said separating zones to reflux condensation in a common refluxing zone, subjecting the resulting vapors to fractionation to separate a clean stock, and cracking said clean stock in said severe cracking zone to produce said hot cracked products.

6. The method of treating hydrocarbon oil comprising cracking a clean oil distillate under a temperature of at least 925 F., discharging the cracked distillate into a high pressure separating zone admixed with reflux condensate from a refiuxing zone maintained at a lower pressure than said separating zone, to rapidly separate vapors from liquids while minimizing further cracking, subjecting the separated hot vapors to reflux condensation with an incoming stream of heavier charging oil in said refluxing zone, to form said reflux condensate, subjecting the liquid from said separating zone to a mild cracking at a temperature of from 850 F. to 950 F., further separating the products of said mild cracking, and removing tar from the system, jointly fractionating the vapors from said refluxing zone and said further separation, and condensing and collecting the fractionated vapors.

7. The method as claimed in claim 6 wherein the pressure maintained in said high pressure separating zone is from pounds to 800 pounds per square inch.

8. The method of treating hydrocarbon 011 comprising cracking a clean oil distillate at a temperature of at least 925 F., discharging the cracked distillate into a high pressure separating zone maintained at a pressure of from 100 pounds to 800 pounds per square inch admixed with reflux condensate from a refluxing zone maintained at a lower pressure than said separating zone, to rapidly separate vapors from liquids while minimizing further cracking, subjecting the separated hot vapors to reflux condensation with an incoming stream of heavier charging oil in said refluxing zone, to form said reflux condensate, said incoming stream having been preheated to a temperature below cracking, subjecting the liquid from said separating zone to a mild cracking at a temperature of from 850 F. to 950 F., further separating the products of said mild cracking, and removing tar from the system, fractionating the vapors from said refluxing zone and said further separation, and condensing and collecting the fractionated vapors.

9 The method of treating hydrocarbon 011 comprising cracking a clean oil distillate, discharging the cracked distillate into a high pressure separating zone admixed with reflux condensate from a refluxing zone maintained at a lower pressure than said separating zone, to rapidly separate by centrifugal action vapors from liquids while minimizing further cracking, subjecting the separated hot vapors to reflux condensation with an incoming stream of heavier charging oil in said refluxing zone, to form said reflux condensate, subjecting the liquid from said separating zone to a mild cracking, further separating the products of said mild cracking, and removing tar from the system, commingling and then fractionating the vapors from said refluxing zone and said further separation, and condensing and collecting the fractionated vapors.

HOWARD V. SMITH. 

